In one approach to materials processing, a substrate is exposed to excited species such as ions or neutral radicals that interact physically or chemically with the substrate so as to effect deposition of material onto or removal of material from the substrate. The excited species are part of an excited gaseous processing medium, for example, a plasma, generated by applying electromagnetic excitation to a reactant gas composition; process parameters such as power level and pressure are chosen to effect the desired process rate and selectivity. Plasma-assisted chemical vapor deposition, plasma ashing and plasma etching, all widely used in the semiconductor industry, embody this approach. For example, in reactive-ion etching systems, both ionic and neutral radical elements participate in the process. Material is removed from the substrate by relatively volatile species created by reaction of radicals with the substrate material; ions impinging the surface provide the energy needed to eject substrate material from the substrate so it can react with a radical, or may volatilize reaction products residing on the substrate.
The plasma is ordinarily generated by applying an oscillating electromagnetic field to a reactant gas composition in order to excite collisions between the molecules that result in ionization or other excitation. Many approaches to applying this excitation have been developed for plasma etching. For the purposes of this disclosure, the phrase "direct-plasma processing" refers to plasma processes in which the substrate is located in an active region containing the plasma, near a powered electrode energized to generate the plasma. For example, in parallel plate reactors, such as are shown in U. S. Pat. Nos. 4,626,312 and 5,248,371, the plasma is generated in situ between a pair of opposing plate electrodes in a diode or triode configuration by a radio frequency ("rf") electrical field oriented perpendicularly to the ;substrate, which is supported by one of electrodes, so that positive ions are accelerated toward it. Variations of this design have incorporated additional electrical or magnetic power sources. For example, Skidmore, Semiconductor International, 1989, pp. 74-79 and U. S. Pat. No. 4,668,338 describe systems incorporating additional magnetic fields for enhancing the plasma density.
By contrast, in so-called "remote-plasma processing", creation of the reactive species is removed from the vicinity of the substrate and does not involve the substrate support. For example, the plasma may be generated in the region of the chamber opposite the substrate by a rf current resonated through a planar coil disposed outside of the reactor chamber. Or, the plasma may be created upstream of a main etching chamber with the reactive constituents being subsequently transported to the main chamber where the etching takes place.
Both types of plasma etching reactors generally incorporate a system, operated in a constant manner during treatment of a series, for removing heat from the substrate. These are present primarily for preventing excessive substrate temperatures in steady-state operation due to, for example, ion bombardment, exothermic chemical reactions or, in direct-plasma processes, waste heat from plasma generation. A mechanism for providing a medium such as inert gas between the substrate and the support is often used to enhance thermal contact between substrate and support. Commonly, a chiller system for receiving excess heat from the substrate support includes a fluid that circulates at a constant rate through the support, thereby receiving heat and carrying it convectively to a refrigeration source, such as a thermostatted liquid bath, to which the heat is transferred. The circulating fluid is usually maintained at a temperature between 0.degree. C. and 500.degree. C. Water is often used as the fluid for these systems; the water circulation rate and properties of the refrigeration source are chosen to maintain the water it a temperature lower than 100.degree. C. throughout the process. For higher temperatures, an alternate fluid such as helium is used. Limiting the substrate temperature is particularly important for etching operations, because the patterning material may incinerate at substrate temperatures higher than about 150.degree. C.
In the manufacture of semiconductor devices, plasma processes arc typically applied to a series of several nominally identical substrates, individually treated in succession by a uniform process. As used in this disclosure, the phrase "uniform process" indicates that each substrate of the series is treated under nominally identical process conditions. A disparity between the process characteristics--for example, etch rate and etch selectivity--of the initial wafers treated by a uniform process and the characteristics of subsequent wafers in the same series has been observed for some plasma processes. This phenomenon has been named the "first-wafer effect." Depending on the characteristic affected and the stringency of its specification, substrates exhibiting the first-wafer effect may be unacceptable, thus reducing the process yield. Although possible causes of the first wafer effect have been investigated, particularly for remote-plasma ashing, process control literature has emphasized accounting or compensating for this effect rather than eliminating it.